The present invention generally relates to thermoreversible organogelator compounds, compositions thereof and methods of forming gels containing these compounds.
In the past, the gelation phenomena, when observed, was a curiosity and mentioned only briefly in the literature. However, in the last 20 years and in particular the last 5-10 years, scientists have begun to understand the forces involved in the gelation process and have designed many molecules that gel everything from alkanes, like hexane, to supercritical CO2. The use of these molecules is as of yet minimal. However, it is clear that by changing the bulk properties of a solvent/solution, commercial applications are possible.
Thermoreversible organogelators generally contain two or more structural domains of differing complementarity. One of these domains is typically a long alkyl chain and the other domain consists of an amide, urea, amino acid, polyaromatic, steroid, calixarene or carbamate group. These domains aggregate to form sheets or rods. Self-assembled aggregates further associate through the alkyl chains to form fibers. The supramolecular fibers further aggregate through the alkyl chains creating junction zones (semi-crystalline sections) between the fibers. This latter form of aggregation establishes a 3-dimensional network, which immobilizes the solvent and sets the gel. Removal of the solvent consistently causes the collapse of the gel and a loss of the three dimensional network resulting in a xerogel.
Previously, a series of thermoreversible gelators for supercritical CO2, such as compounds 1 and 2, as shown below, have been prepared. Upon removal of the solvent they form microcellular foams which are aerogels, materials that retain the three dimensional network in the absence of a solvent, in contrast to xerogels. The determination of the physical structure of these foams was achieved through scanning electron microscopy (SEM). The morphology of the gels is highly dependent on the structure of the monomer unit comprising the aggregate as well as the concentration under which the aggregate is allowed to form. Examination of the SEM's provides details of the modes of aggregation and allows correlation of the structure of the gel to the thermodynamic behavior.
Certain bis-urea organogelators have been shown to aggregate
through complementary bidentate intermolecular hydrogen bonds where both the NH and C═O groups of the ureas participate in the aggregate growth as evidenced by the IR spectrum. Molecular modeling studies of compound 3 predict that the primary aggregate has rod-like structures (while compound 4 forms sheet-like structures) which assemble into fibers. The structure of compounds 3 and 4 as well as their proposed hydrogen-bonding pattern are shown below. Further evidence for rod formation by self-complementary hydrogen bonding of bis-ureas has been obtained from a single crystal X-ray structure of bis-urea compounds. Further evidence for stacking and a degree of chiral recognition has been observed by CD measurements of an azobenzene derivative of compound 3.

Aggregation through the bis-urea motif creates gels at a lower concentration as compared to other organogelators, due to the structural integrity of the primary aggregate. The stronger the initial aggregate, the longer the rod or sheet can grow, increasing the van der Waals surface area of the aggregate, which is an important factor in determining gel stability. Disruption of this initial aggregate shortens the length of the supramolecular complex, thereby creating less surface area for cross-linking. This shortening decreases the size of the junction zones and changes the thermodynamic behavior of the system.
Thermal stability of the gel relies not only on the strength of the initial aggregation but also on the degree of cross-linking, which is caused by the alkyl tails. Typically, the longer the alkyl tail (larger the van der Waals surface area) the more thermal stability the gel possesses. Junction zones formed from the alkyl tails grow over time due to thermal motion. This leads to time dependence as well as temperature dependence in the thermal stability of the gel and, as a result, the enthalpy of melting is also dependent on these variables.
Additional work has been done to increase the thermal stability of organogels by incorporating covalent crosslinks. One method that has been explored by others in the field is to chemically crosslink the monomeric organogelator after the gel has set. The increased stability comes from the covalent bonds created between the methacrylate fragments on the molecule. Compound 5, as shown below, has an increased thermal stability after photopolymerization of ethyl acetate gels since the pre-irradiation melting temperature of the gel is 40° C. while after irradiation, is more than 110° C. This increase is impressive since ethyl acetate boils at 77° C. However, this degree of stability is not uncommon in polymerized gels. Locking the structure by polymerization gives rise to aerogels upon solvent removal. But since the structure is now covalently crosslinked, it is no longer thermoreversible.

In addition to bis-ureas, bis-semicabazides like compound 6, as shown below, are known to gel organic liquids. This class of gelator has been shown to gel organic liquids in the range of 20 wt % to 0.6 wt % depending on the solvent and the exact gelator used. Of particular note is that this type of gelator when dissolved in N,N-dimethylacetamide (DMA) at 33 wt % can be used to gel less polar solvents. By diluting the gelator-DMA mixture with a less polar solvent, a gel will form and heating of the less polar solvent is avoided.

The same principle and forces that are utilized in organogelation are used for hydrogelation (gelation of water). Like organogelation, work in this area is extensive and has been ongoing for a significantly longer time. So far, there are only two examples of hydrogelators: a monourea of compound 7 and a bis-urea of compound 8, both shown below. These molecules have ionizible side chains that can hydrogen bond with water and impart a low degree of solubility in warm water. Critical concentrations, the minimum amounts of the compound in a solvent at which fluidic motions of the solvent stops, vary from 0.8 to 2 wt %. Compound 7 is one of the smallest low molecular weight hydrogelators known to date. Compound 8 was initially designed as a model system for the biomineralization of calcite.

Therefore, it is a challenge to provide a compound that is stable, is disruptable or thermoreversible when it encloses a variety of solvents to form a gel composition, and has a low critical concentration. Using a low critical concentration for the compound to immobilize a solvent is cost efficient because a small amount of the compound is needed to form a gel.